1. Field of the Invention
This invention relates to a circuit which reduces the turn-off time of a power transistor.
2. Description of the Prior Art
When power transistors are used as the switching means to drive a heavy load, such as in pulse width modulated servo systems, upon discontinuance of the drive signal to the base of the power transistor, the power transistor does not turn off immediately due to the well known time delay storage effect present in all transistor devices. As soon as the storage delay has lapsed, the voltage across the collector to emitter of the transistor begins to rise. However, the transistor is still at this time drawing significant load current. Thus, the power dissipated by the transistor is at a maximum value. In order to insure that the transistor will not fail due to the generation of heat when the transistor is turning off, it is often necessary to encumber the equipment with elaborate heat sinking means to further dissipate the heat from the transistor. Such heat sinking means at times will increase the size and cost of equipment beyond permissable bounds. At other times, such heat sinking means may not be sufficient to avoid the failure of the power transistor, and the system may have to operate at substantially reduced load current levels.